1. Field of the Invention
The present invention relates to a method for producing a semiconductor device using a thin film of a crystalline semiconductor, and particularly, to a method for producing a thin film transistor.
2. Description of the Related Art
Recently, much attention is paid on transistors which utilize a thin film of a semiconductor formed on a glass or quartz substrate. Such thin film transistors (TFTs) are fabricated by forming a thin film semiconductor from several hundreds to several thousands of angstroms (.ANG.) in thickness on the surface of a glass substrate or a quartz substrate, and then forming a transistor (insulated gate field effect transistor) using the thin film semiconductor.
TFTs are used in an application field such as that of an active matrix type liquid crystal display device. An active matrix type liquid crystal display device has several hundred thousands of pixels arranged in a matrix, and TFTs are provided to each of the pixels as switching elements to realize a fine and a high speed display. Practically available TFTs designed for an active matrix type liquid crystal display device utilize thin films of amorphous silicon.
However, TFTs based on thin films of amorphous silicon are still inferior in performance. If a higher function is required as a liquid crystal display of an active matrix type, the characteristics of TFTs utilizing an amorphous silicon film are too low to satisfy the required level.
Furthermore, it is proposed to fabricate an integrated liquid crystal display system on a single substrate by using TFTs; i.e., by realizing not only the pixel switching, but also the peripheral driver circuit with TFTs. However, a TFT using an amorphous silicon thin film cannot constitute a peripheral driver circuit because of its low operation speed. In particular, a basic problem is that a CMOS circuit is unavailable from an amorphous silicon thin film due to the difficulty in implementing a practical P-channel type TFT by using amorphous silicon thin film (i.e., the TFT using amorphous silicon thin film is practically unfeasible due to its too low performance).
Another technology is proposed to integrate other integrated circuits and the like for processing or recording image data, etc., on a single substrate together with the pixel regions and the peripheral driver circuits. However, a TFT using a thin film of amorphous silicon is too inferior in characteristics to constitute an integrated circuit capable of processing image data.
On the other hand, there is a technology of fabricating a TFT using a crystalline silicon film which is far superior in characteristics as compared with the one using a thin film of amorphous silicon. The technology comprises forming a film of amorphous silicon and then modifying (transforming) the resulting film of amorphous silicon to a crystalline silicon film by subjecting the amorphous silicon film to thermal treatment or to laser irradiation. The crystalline silicon film thus obtained by crystallizing the amorphous silicon film generally yields a polycrystalline structure or a microcrystalline structure.
As compared with a TFT using an amorphous silicon film, a TFT having far superior characteristics can be implemented by using a film of crystalline silicon. Concerning mobility, which is one of the indices for evaluating TFTs, a TFT using amorphous silicon film has 1 to 2 cm.sup.2 /Vs or lower (in an N-channel type), but a TFT using a crystalline silicon film enables a mobility of about 100 cm.sup.2 /Vs or higher in an N-channel type, or about 50 cm.sup.2 /Vs or higher in a P-channel type.
The crystalline silicon film obtained by crystallizing an amorphous silicon film has a polycrystalline structure, and hence various problems attributed to the grain boundaries arise. For instance, carriers which move through the grain boundaries greatly limit the withstand voltage of the TFT. The change or degradation in characteristics easily occurs in high speed operation. Further, the carriers which move through the grain boundaries increase the OFF current (leak current) when the TFT is turned off.
In fabricating a liquid crystal display device of an active matrix type in a higher integrated constitution, it is desired to form not only the pixel region but also the peripheral circuits on a single glass substrate. In such a case, it is required that the TFTs provided in the peripheral circuit operate a large current to drive several hundred thousands of pixel transistors arranged in the matrix.
A TFT of a structure having a wide channel width must be employed to operate a large current. However, even if the channel width should be extended, a TFT using a crystalline silicon film cannot be put into practice because of the problems of withstand voltage. The large fluctuation in threshold voltage is another hindrance in making the TFT practically feasible.
A TFT using a crystalline silicon film cannot be applied to an integrated circuit in processing image data because of problems concerning the fluctuation in threshold voltage and the change in characteristics with passage of time. Accordingly, a practically feasible integrated circuit based on the TFTs which can be used in the place of conventional ICs cannot be realized.